Hypothyroidism is one of the most common endocrinopathies in dogs. The diagnosis of hypothyroidism relies upon measurement of basal plasma concentrations of total thyroxine (T₄) and thyroid stimulating hormone (TSH). A low plasma T₄ combined with a high plasma TSH points to primary hypothyroidism. However, 30%–38% of hypothyroid dogs have TSH values within the reference range. Consequently, a low plasma total T₄ concentration in combination with a plasma TSH concentration within the reference range does not distinguish between dogs with hypothyroidism and dogs with non-thyroidal illness (NTI). A potential interesting observation from previous studies (Lee et al. 2001, and Diaz-Espiñeira et al. 2008) is that hypothyroidism in dogs is associated with increased release of growth hormone (GH).

The aim of this study was to evaluate whether a TRH stimulation test can differentiate between dogs with NTI and dogs with hypothyroidism that have a plasma TSH concentration within the reference range, by measuring plasma concentrations of GH and TSH.

Twenty-one dogs with clinical signs consistent with hypothyroidism, a plasma TT4 concentration below the reference interval (19–46 nmol/l), and a plasma TSH concentration within the reference interval (<0.60 µg/l) were included in this study. Thyroid scintigraphy was performed to classify dogs as having hypothyroidism or NTI. All dogs underwent a TRH stimulation test in which plasma concentrations of TSH and GH were measured twice before intravenous administration of 10 µg/kg TRH (t=-15 and t=0) and 30 and 45 minutes after TRH administration.

Eleven of the 21 dogs were classified as hypothyroid and 10 dogs as having NTI by thyroid scintigraphy. There were no differences in baseline characteristics between the groups except for gender. The plasma TSH concentration did not change significantly in the hypothyroid dogs after administration of TRH, whereas it significantly increased in the NTI dogs (p<0.001). In contrast, the plasma GH concentration increased significantly in the hypothyroid dogs after TRH administration (p=0.009), whereas it did not change in the NTI dogs.

The TRH stimulation test with measurement of circulating concentrations of TSH and GH could be used to differentiate between hypothyroid dogs and NTI dogs that have clinical signs of hypothyroidism, a low basal TT4 concentration and a basal TSH concentration within the reference interval. This is a promising test which might be of valuable use in primary veterinary practice.

Disclosures

Disclosures to report:

This study was partially funded by a grant from the American Kennel Club.